Defender Wayne Thomas was named as a substitute after signing a one-year deal. Youngster Ryan Mason was handed his debut after he joined the club on loan Friday afternoon.
Rovers' club record signing Sharp, a 1.1million capture from Sheffield United in July, blasted home his first goal of the season from the spot after Jamie McAllister's 89th-minute foul on Doncaster substitute James Coppinger.
It was a cruel blow for City, who had taken the lead early in the second half when Damion Stewart headed home McAllister's cross, in Keith Millen's first game in charge as permanent manager.
City wasted little time in appointing Millen as Coppell's successor after the former Crystal Palace and Reading boss announced his decision to quit football management on Thursday and the Robins responded admirably after successive defeats had made for a miserable start to their season.
But despite playing second fiddle for most of the second half, Sean O'Driscoll's Rovers were rewarded for a spirited finish.
O'Driscoll felt his side were good value for a point and praised record signing Sharp for holding his nerve to beat David James from the penalty spot.
"I thought we were the better side by far in the first half. All the things you want your players to do we were doing." said O'Driscoll.
"But I was disappointed with the reaction from everybody - players and crowd - after they scored.
"We weren't brave enough at times to show the patience we required. The crowd want us to smash it forward sometimes but if we're going to do that we might as well walk off the pitch because that's not the way we play."
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